Aerospace sealants must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. Sealants comprising sulfur-containing prepolymers that exhibit acceptable fuel resistance, thermal resistance, and flexibility for aerospace applications are described, for example, in U.S. Pat. No. 6,172,179. In sealants such as those described in U.S. Application Publication Nos. 2006/0270796, 2007/0287810, and 2009/0326167, a sulfur-containing polymer such as a thiol-terminated polythioether prepolymer can be reacted with a polyepoxide curing agent in the presence of an amine catalyst to provide a cured product. These systems are useful as sealants and can meet the demanding performance requirements of the aerospace industry. Without a strong base catalyst, such as an amine catalyst, the reaction between the thiol and epoxy groups is slow and provides a relatively long working time, for example, of several days to weeks, depending on the temperature. However, the physical properties of these slowly cured sealants are generally not acceptable. In contrast, in the presence of a strong base catalyst the reaction is fast, and although exhibiting acceptable cured properties, provides a working time of only about 2 hours to about 12 hours depending upon the particular system. For many applications, however, a longer working time such as from 12 hours to 48 hours is desirable.
In practice, sealants can be provided as two-part compositions in which a thiol-terminated sulfur-containing prepolymer and a polyepoxide are provided as separate components, with the amine catalyst in the thiol-containing component, and the two parts can be mixed shortly prior to use. Alternatively, the base catalyst may be provided as a third component, and the component containing the thiol-terminated sulfur-containing prepolymer, the component containing the polyepoxide, and the component containing the base catalyst can be mixed shortly before use. However, once the components are mixed, the thiol and epoxy groups react, and depending at least in part on the temperature and on the type of amine catalyst, the working time can be limited to less than from 2 hours to 12 hours. Furthermore, once the reactants are combined and as the composition cures, there is little ability to control the reaction rate to take advantage of the complex chemistries taking place after the sealant is applied to a surface.
As disclosed in U.S. Pat. No. 9,006,360, compositions comprising thiol-terminated sulfur-containing prepolymers, polyepoxides, and an encapsulated amine catalyst provide an extended working time. However, the components forming the encapsulant, which become incorporated into the cured polymer network, can compromise properties such as the fuel resistance of the cured polymer.
Ways to extend the working time and to control the curing rate of compositions comprising thiol-terminated sulfur-containing prepolymers and polyepoxides and that provide a cured sealant that meets the performance requirements of aerospace applications are desired.